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<br />Gunnison county. All bridges and culverts were measured 1n the <br />field to obtain elevation data and structural geometry. <br /> <br />Locations of selected cross sections used in the hydraulic analy- <br />ses are shown on the Flood Profiles (Exhibit 1). <br /> <br />Roughness factors (Manning' s "n") were determined by field inspec- <br />tion. Ground level photography aided in the selection of rough- <br />ness factors and orthophoto contour maps were used to determine <br />the limits of each roughness zone (References 6 and 7). Roughness <br />values for the main channel of the streams studied by detailed <br />methods ranged from 0.030 to 0.039 with floodplain values ranging <br />from 0.032 to 0.085 for all floods (Reference 8). <br /> <br />Starting water-surface elevations for the Gunnison River were <br />based on discharge rating curves for the Gunnison River near <br />Gunnison gaging stations and utilization of the slope area method <br />(Reference 2). The starting water-surface elevation for the North <br />Fork Gunnison River was based on information provided by the CWCB <br />and the Commissioners of Gunnison County (Reference 1). <br /> <br />Flood profiles were drawn showing the computed water-surface ele- <br />vations for floods of the selected recurrence intervals. <br /> <br />The hydraulic analyses for this Flood Insurance Study are based on <br />unobstructed flow. The flood elevations shown on the profiles are <br />thus considered valid only if hydraulic structures remain <br />unobstructed, operate properly, and do not fail. <br /> <br />All elevations are referenced to the National Geodetic Vertical <br />Datum of 1929. <br /> <br />4.0 FLOODPLAIN MANAGEMENT APPLICATIONS <br /> <br />The NFIP encourages State and local governments <br />plain management programs. Therefore, each <br />includes a floodplain boundary map designed to <br />developing sound floodplain management measures. <br /> <br />to adopt sound flood- <br />Flood Insurance Study <br />assist communities in <br /> <br />4.1 Floodplain Boundaries <br /> <br />To provide a national standard without regional discrimination, <br />the I-percent annual chance (lOO-year) flood has been adopted by <br />FEMA as the base flood for floodplain management purposes. The <br />0.2-percent annual chance (SOO-year) flood is employed to indicate <br />additional areas of flood risk in the community. For each stream <br />studied by detailed methods, the boundaries of the 100- and 500- <br />year floodplain boundaries have been delineated using the flood <br />elevations determined at each cross section. <br /> <br />9 <br />